Vanadium redox flow batteries (V-RFB) are a representative flow battery, and a secondary battery in which vanadium is used as a positive electrode and negative electrode electrolyte and the charge/discharge occurs due to the oxidation and reduction reactions of the electrolyte. The biggest difference from general batteries is that the charge/discharge occurs while circulating the electrolyte where energy is stored. The charge/discharge occurs in a stack where the electrochemical reactions of oxidation and reduction occur, and electricity is stored in an electrolyte kept in a separate tank.
Therefore, the vanadium redox flow battery (V-RFB) has advantages such as capacity enlargement, economic efficiency, and stability, and thus has been highlighted as a next-generation energy storage device. In order to increase the energy density (kW/L) of the electrolyte, it is essential to develop a high concentration of vanadium electrolyte, but there is a problem in that vanadium precursors exhibit low solubility in an aqueous sulfuric acid solution.
The existing general vanadium electrolyte is prepared by a process in which sulfuric acid is introduced into distilled water to prepare a 2 to 3 M aqueous sulfuric acid solution, and a vanadium precursor is introduced into the prepared aqueous sulfuric acid solution.
This process is known as a method used in the dissolution of usually a less than 1.5 M vanadium precursor when considering the stability against precipitation.
In the preparation of the vanadium electrolyte, there are disadvantages in that a vanadium ion in the vanadium electrolyte may combine with oxygen to form a complex compound due to dissolved oxygen that is dissolved in distilled water, and when the concentration of the aqueous sulfuric acid solution is 3 M or more, the solubility of the vanadium precursor is reduced.
Recently, studies have been continuously conducted on the development of a high concentration of a vanadium electrolyte, but most of the studies have been limited to studies in which carboxylic acid, EDTA, a metal salt, an ammonium salt, glycerin, and alcohols are introduced as an additive to suppress the precipitation of vanadium salts, increase the dispersion stability, or develop an electrolytic method having a complicated process.
In the case of a method which uses an additive, through the interaction of the additive with the vanadium ion, the solubility of a vanadium precursor may be increased and the precipitation rate and the precipitation reaction may be suppressed, but there is a problem in that in the battery reaction, the oxidation and reduction reversible reaction rate of the vanadium ion is reduced, and there is a disadvantage in that at the battery operating voltage, the additive may be electrolyzed to cause a drop in battery voltage and a decrease in capacity or generate gases such as carbon dioxide and hydrogen gases.